Utility meters are devices that, among other things, measure the consumption of a utility provided commodity, such as electric energy, gas, or water, by a residence, factory, commercial establishment or other such facility. Utility service providers, or simply utilities, employ utility meters to track individual customers' usage of utility provided commodities. Utilities track customer usage for many purposes, including billing and tracking demand for the relevant consumed commodity.
Utilities are frequently required by state or local regulations or by customers to verify the accuracy of meters operating in the field. To satisfy these requirements, utility meters are typically subjected to stringent testing and calibration during and after manufacture. In general, if the results of testing indicate that a given meter is not operating at a desired degree of accuracy, the meter may be calibrated or adjusted to bring the meter within the desired tolerances.
One factor affecting the accuracy of utility meters is ambient temperature. In particular, ambient temperature variations often affect a meter's ability to measure time accurately. This is particularly significant since meters typically use time as a basis for measuring customer usage of utility provided commodities. Accordingly, time measurement errors may cause a meter to generate faulty customer usage information, thereby resulting in customer overbilling or underbilling.
Examples of time-based metering operations include demand metering and time-of-use metering, which are commonly used in electricity metering. Demand metering is a type of revenue metering that measures the amount of energy used in discrete increments of time, for example, every half-hour or hour, and then bases the cost of electrical energy on the highest demand intervals. Time of use metering employs different energy rates at different times of the day. For example, electrical energy may cost more during the afternoon in the summer months than at night during the winter months.
Demand metering and time-of-use metering thus require fairly accurate time reference clocks, for example, real time calendar/clocks, to ensure that the billing amounts are fair and accurate. Time reference clocks may likewise be employed for other functions in electricity meters as well as other types of meters. Accuracy of such clocks is clearly desirable.
Many electricity meters for the U.S. market have historically used the 60 Hz power line frequency as a time reference since it is synchronized to the National Bureau of Standards and exhibits reasonably high accuracy. However, line power is not available during a power outage. In the past, when meters only measured total customer usage of a commodity, the use of the line frequency as a time reference was sufficient since no data was acquired during the loss of power, and therefore, no time reference was required.
With the advent of electronic meters, however, and the consequent increase in use of real time clock information, there is an increased requirement to maintain a time keeping function during power outages so that when power is restored, the time and date references within the meter are accurate. To keep time during power outages, electricity meters often employ crystal-based clocks.
Crystal oscillators, however, are susceptible to operational variations based on ambient temperature variations. Such operational variations can result in significant inaccuracies in time-keeping even during short duration power outages. Moreover, for non-electricity meters that rely on crystal oscillators full-time, inaccuracies could be substantial.
Accordingly, there is a need for a utility meter capable of compensating for ambient temperature variations in order to, among other things, maintain time keeping accuracy. The present invention addresses these and other issues.